Extracellular heat shock protein 70 inhibits tumour necrosis factor-α induced proinflammatory mediator production in fibroblast-like synoviocytes
© Luo et al.; licensee BioMed Central Ltd. 2008
Received: 28 December 2007
Accepted: 14 April 2008
Published: 14 April 2008
It was recently suggested that heat shock protein (HSP)70, an intracellular protein, is a potential mediator of inflammatory disease when it is released into the extracellular compartment. Although elevated HSP70 levels have been identified in rheumatoid arthritis (RA) synovial tissues and RA synovial fluid compared with patients with osteoarthritis and healthy individuals, it remains unclear what role extracellular HSP70 plays in the pathogenesis of RA. This study was conducted to investigate the effects of extracellular HSP70 on the production of RA-associated cytokines in fibroblast-like synoviocytes from patients with RA and to elucidate the mechanisms involved.
IL-6, IL-8 and monocyte chemoattractant protein (MCP)-1 levels in culture supernatants were measured using enzyme-linked immunosorbent assays. Activation of mitogen-activated protein kinases (MAPKs), such as extracellular signal-regulated protein kinases (ERKs), c-Jun amino-terminal kinase (JNK) and p38 MAPK, was detected using Western blotting. Nuclear translocation of nuclear factor-κB (NF-κB) and degradation of the inhibitory protein IκBα were examined using immunohistochemistry and Western blotting.
Human HSP70 downregulated IL-6, IL-8 and MCP-1 production in RA fibroblast-like synoviocytes induced by tumour necrosis factor (TNF)-α in a concentration dependent manner. HSP70 inhibited the activation of ERK, JNK and p38 MAPK in fibroblast-like synoviocytes stimulated by TNF-α. Furthermore, HSP70 also significantly inhibited nuclear translocation of nuclear factor-κB and degradation of IκBα induced by TNF-α.
Extracellular HSP70 has an anti-inflammatory effect on RA by downregulating production of IL-6, IL-8 and MCP-1 in fibroblast-like synoviocytes, which is mediated through inhibited activation of the MAPKs and NF-κB signal pathways.
Rheumatoid arthritis (RA) is a chronic disease that is characterized by inflammation of the synovial membrane and proliferation of the synovial lining, resulting in progressive joint destruction . Fibroblast-like synoviocytes (FLSs) play a crucial role in the joint inflammation and destructive process . RA FLSs respond to several proinflammatory cytokines, including IL-1, tumour necrosis factor (TNF)-α, and exhibit characteristics of inflammatory cells that are critically involved in various aspects of rheumatoid pathophysiology [2, 3]. They synthesize and secrete proinflammatory cytokines such as IL-6, and chemokines including IL-8 and monocyte chemoattractant protein (MCP)-1 [4–7], which play roles in mediating the inflammatory functions of FLSs. IL-6 is now recognized to be a master cytokine that is involved not only in the RA cytokine cascade but also in actions such as promotion of expansion and activation of T cells, differentiation of B cells, regulation of acute phase protein genes, and regulation of chemokine production [8, 9]. IL-8 and MCP-1 are key mediators that are involved in the recruitment of neutrophils, monocytes and lymphocytes, and play important roles in inflammatory cell infiltration . Evidence from animal models of arthritis and from RA patients has shown that blockade of these cytokines or their receptors has beneficial effects both for inflammation and joint destruction [10–12]. Therefore, inhibition of these inflammatory mediators production by FLSs might present an effective target for RA treatment.
Heat shock proteins (HSPs) are a family of highly conserved intracellular proteins that are found in all prokaryotes and eukaryotic cells. Although some HSPs are constitutively expressed, upregulation of expression is caused by exposure to a variety of cellular stressors, including heat shock, growth factors, inflammation and infection [13, 14]. HSPs are typically regarded as intracellular proteins, and their primary function appears to be that of intracellular molecular chaperones, contributing to the folding of nascent proteins and denatured proteins, and thus preventing protein aggregation under stressful stimuli [15, 16]. The human stress-inducible form of the 70 kDa HSP (HSP70; Genbank: NM005345) is a many-faceted molecule. In addition to serving as a intracellular chaperone, it is released from damaged cells or viable cells after stress, and has been found in the bloodstream of both healthy individuals and those suffering from autoimmune diseases and inflammatory conditions [17, 18]. Recent findings indicating a role for extracellular HSP70 as a cytokine that induces secondary proinflammatory cytokine (TNF-α, IL-1 and IL-6) production may provide insight into the pathogenesis of autoimmune disease [16, 19].
Elevated levels of the inducible form of HSP70 have been identified in RA synovial tissues and RA synovial fluid relative to those in patients with osteoarthritis and healthy individuals [20, 21]. It is unknown whether an increase in extracellular HSP70 plays a biological role in RA, but in animal models pre-immunization with proteins of the HSP70 family, such as mycobacterial HSP70 and the glucose-regulated protein 78, protected animals from experimentally induced arthritis. In adjuvant-induced arthritis in rats it was shown that the protection conferred by mycobacterial HSP70 resulted from the induction of IL-10 producing T cells that were capable of downregulating inflammation [22–24]. However, the precise mechanism of protection by HSP70 in RA remains unclear. Findings in arthritis models raise the question of whether HSP70 could play a role in FLSs. Analyses of FLSs from human patients with RA reveal significantly elevated extracellular expression of HSP70 , which suggests that extracellular HSP70 may play an immunomodulatory role in FLSs. However, the interactions of HSP70 with FLSs in RA have not previously been reported. Also unknown are whether HSP70 exogenously regulates production by FLSs of RA-associated proinflammatory mediators.
In the present study we report the first analysis of the effects of extracellular human inducible HSP70 on TNF-α induced secretion by RA FLSs of the proinflammatory cytokine IL-6 and the chemokines IL-8 and MCP-1, and we elucidate the underlying mechanism. We find that human HSP70 inhibits TNF-α induced IL-6, IL-8 and MCP-1 secretion by human RA FLSs. Furthermore, we demonstrate that human HSP70 suppresses the activation of nuclear factor-κB (NF-κB) and mitogen-activated protein kinase (MAPK) signalling pathways induced by TNF-α. These findings clearly reveal an anti-inflammatory effect of human HSP70 on TNF-α-mediated inflammation and demonstrate its mechanism in RA FLSs.
Materials and methods
FLSs were isolated from RA synovial tissues obtained at joint replacement surgery, as previously described . The diagnoses of RA conformed to the 1987 revised American Council of Rheumatology criteria . Briefly, tissue samples were minced and treated with 1 mg/ml collagenase for 1 to 2 hours at 37°C. After washing, the cells were cultured in Dulbecco's modified Eagle's medium (Life Technologies, Rockville, MD, USA) supplemented with 10% heat-inactivated foetal calf seum (Life Technologies), 100 IU/ml penicillin, 100 μg/ml streptomycin, and 2 mmol/l L-glutamine in a humidified incubator with 5% carbon dixoide and 95% air. After overnight culture nonadherent cells were removed, and adherent cells were cultivated in Dulbecco's modified Eagle's medium plus 10% foetal calf seum. At confluence, cells were trypsinized, split at a 1:3 ratio and re-cultured in the same medium. All the experiments described here utilize FLSs between the fourth and ninth passage. That the population of FLSs was homogeneous was determined using flow cytometry (<1% CD11b, <1% phagocytic and <1% Fcγ receptor type II positive).
Cytokine quantification by ELISA
Following stimulation of human RA FLSs by TNF-α (R&D, Minneapolis, MN, USA) in the presence or absence of recombinant human inducible HSP70 (StressGen, Victoria, British Columbia, Canada; catalog# ESP555), supernatants were harvested. IL-6, IL-8 and MCP-1 levels were measured using a sandwich ELISA, following the manufacturer's instructions (R&D). All data were normalized by cell number.
Preparation of cellular extracts
Following treatment with TNF-α, cells were harvested, washed twice with cold phosphate-buffered saline (PBS), and nuclear and cytoplasm extracts were prepared in accordance with the method proposed by Edgar and coworkers . Briefly, the cell pellet was resuspended in 200 μl cold buffer A (10 mmol/l HEPES [pH 7.9], 10 mmol/l KCl, 0.1 mmol/l EDTA, 0.1 mmol/l EGTA, 1 mmol/l DTT and 0.5 mmol/l phenylmethylsulphonyl fluoride [PMSF]). The cells were allowed to swell on ice for 15 minutes, after which 25 μl of a 10% solution of NP-40 was added and the tube was vortexed for 10 seconds. The homogenate was centrifuged for 30 seconds in a microfuge to recover the cytoplasm extract in the supernatant. The nuclear pellet was resuspended in 50 μl ice-cold buffer B (20 mmol/l HEPES [pH 7.9], 0.4 mol/l NaCl, 1 mmol/l EDTA, 1 mmol/l EGTA, 1 mmol/l DTT and 1 mmol/l PMSF) and the tube was vigorously rocked at 4°C for 15 minutes on a shaking platform. The nuclear homogenate was centrifuged for 5 minutes to recover the nuclear extract in the supernatant. The aliquots were stored at -80°C. The protein concentrations of the fractions were determined using a standard Bradford assay.
Western blot analysis
The following antibodies were used in this study: rabbit anti-phospho-JNK (c-Jun amino-terminal kinase) polyclonal antibody (Cell Signaling, Danvers, MA, USA; 1:1000); rabbit anti-phospho-p38 polyclonal antibody (Cell Signaling; 1:1000); sheep anti-phospho-Erk1/2 (extracellular signal-regulated protein kinase-1/2) polyclonal antibody (Upstate, Lake Placid, NY, USA; 1:1000); rabbit anti-NF-κB (nuclear factor-κB) p65 polyclonal antibody (Santa Cruz, Santa Cruz, CA; 1:200); rabbit anti-JNK polyclonal antibody (Cell Signaling; 1:1000); rabbit anti-p38 polyclonal antibody (Cell Signaling; 1:1000); rabbit anti-Erk1/2 polyclonal antibody (Cell Signaling; 1:1000); rabbit anti-IκBα polyclonal antibody (Santa Cruz; 1:1000); mouse anti-GAPDH (glyceraldehyde 3-phosphate dehydrogenase) monoclonal antibody (Upstate; 1:1000); and mouse anti-PCNA (proliferating cellular nuclear antigen) monoclonal antibody (Upstate; 1:1000), and mouse anti-tublin monoclonal antibody (Upstate; 1:1000).
Aliquots of either subcellular fractions or total cell lysates were reduced and denatured by boiling in SDS sample buffer (2× = 100 mmol/l Tris-HCl [pH 6.8]; 4% weight/vol SDS; 20% glycerol; 200 mmol/l DTT; 0.1% weight/vol bromophenol blue), fractionated on 12% SDS-PAGE, and transferred to a nitrocellulose membrane (Promega, Madison, WI, USA). Membranes were blocked in blocking buffer (2% bovine serum albumin [BSA], 0.2% Tween-20 in Tris-buffered saline) at room temperature for 4 hours, and incubated for 2 hours at 25°C with the indicated primary antibody, followed by peroxidase-conjugated secondary antibody IgG (anti-rabbit or anti-mouse [Boster Biotech, Wuhan, China; 1:1000], anti-sheep [KPL, Gaithersburg, MD, USA; 1:1000]) for 1 hours at 25°C. The signals were visualized by DAB detection (Boster Biotech), following the manufacturer's instruction, and the bands of interest were scanned and counts quantitated with the Band Leader software (Shanghai, China).
FLSs (5 × 105 cells), cultured on glass coverslips, were fixed in 4% formaldehyde for 30 minutes at room temperature before detergent extraction with 0.1% Triton X-100 for 10 minutes at 4°C. Coverslips were blocked in PBS containing 2% BSA for 1 hour at room temperature and processed for immunofluorescence with rabbit anti-NF-κB/p65 polyclonal antibody (Santa Cruz Biotechnology, Inc., Santa Cruz, CA, USA; 1:50) followed by Cy3-conjugated sheep anti-rabbit IgG (Santa Cruz; 1:100) and Hoechst 33258 (Sigma, St. Louis, MO, USA; 1 μg/ml). Between all incubation steps, cells were washed three times for 30 minutes with PBS containing 0.2% BSA. Coverslips were mounted on slides using Movio (Sigma). Fluorescence signals were analyzed by fluorescent microscopy (Nikon, Tokyo, Japan).
Data in the figures and text were expressed as means ± standard deviation. P < 0.05 was deemed to represent statistical significance, and the significance of differences between groups was determined using two-tailed Student's t-test or Fisher's least significant difference test.
Human HSP70 inhibits TNF-α induced IL-6, IL-8 and MCP-1 secretion in FLSs
Human HSP70 suppresses the phosphorylation of MAPKs induced by TNF-α in FLSs
Human HSP70 inhibits nuclear translocation of NF-κB induced by TNF-α in FLSs
Human HSP70 inhibits the TNF-α induced degradation of IκBα
In order to examine the roles played by IκBα in the NF-κB activation pathway by masking the nuclear localization sequence, we investigated the degradation of IkBα by Western blotting using antibodies against IκBα. To explore the mechanism by which HSP70 inhibits the nuclear translocation of NF-κB, we determined whether HSP70 could inhibit the TNF-α induced degradation of IκBα. TNF-α (20 ng/ml) markedly induced degradation of IκBα at 20 minutes, and this degradation was significantly inhibited in cells pretreated with human HSP70 (Figure 6b). It was therefore concluded that HSP70 can inhibit the degradation of IκBα and subsequent nuclear translocation of NF-κB induced by TNF-α.
It has been demonstrated that the HSP70 family of proteins can downregulate adjuvant arthritis , and it is likely that the protection resulted from induction of self-HSP70 cross-reactive T cells that are capable of downregulating inflammation . However, the mechanism underlying the regulatory effect of HSP70 in RA is complex and incompletely understood. Synovial FLSs play a vital role in both chronic inflammation and joint destruction, principally through synthesis of proinflammatory cytokines and chemokines , which play essentially pathogenetic roles in RA. In the present study we investigated – for the first time – the effects of human HSP70 on secretion of the proinflammatory cytokine IL-6 and chemokines IL-8 and MCP-1 by human RA FLSs. Our results clearly demonstrated that human HSP70 suppressed TNF-α induced IL-6, IL-8 and MCP-1 production in a dose-dependent manner in human RA FLSs. Moreover, we observed that HSP70 suppressed the activation of the proinflammatory mediator associated NF-κB and MAPKs signalling pathways induced by TNF-α. Based on these combined observations, we conclude that extracellular human HSP70 has anti-inflammatory effects on RA, probably due to inhibitory effects of HSP70 on production of proinflammatory cytokines and chemokines in FLSs.
The roles played by HSP70 in the immune response have been a focus of many recent investigations [24, 37]. HSP70 has been reported to have both proinflammatory and anti-inflammatory effects in autoimmune diseases . Mammalian and bacterial HSP70 have been described to activate antigen-presenting cells directly, including macrophages and dendritic cells . Such immune activation might contribute to breaking of tolerance to autoantigens, leading to the induction of autoimmune disease . HSP70 can also regulate autoimmunity indirectly by activating regulatory T cells that control pathogenic T cells specific for self-antigens other than HSPs . HSP70 has also been studied and identified as an immune target in RA. The HSP70 family of proteins has been implicated in the pathogenesis of experimental and human arthritis. Immunization with mycobacterial HSP70 has been found to protect rats from experimentally induced arthritis through induction of IL-10 producing T cells [22, 23, 38]. Intravenous or subcutaneous administration of the endoplasmic reticulum chaperone BiP (a mammalian HSP70 family member) was also found to prevent induction of and to treat ongoing collagen-induced arthritis . Elevated levels of antibody to HSP70 have been reported in RA, and the level of HSP70 has been shown to be enhanced in RA synovial fluid and synovial tissue [20, 21, 40].
Although studies on the role of extracellular HSP70 in human RA are incomplete, a picture is emerging in which the expression of HSP70 or immune reactivity to HSP70 in RA appears to be associated with downregulation, rather than induction or propagation of inflammation. It was recently shown that mycobacterial HSP70 treatment in vitro induced IL-10 production in monocytes from blood and synovial tissue from arthritis patients . Also, BiP stimulation of human peripheral blood mononuclear cells in vitro was found to trigger the production of anti-inflammatory cytokines . However, to our knowledge, the effect of extracellular human HSP70 on human RA FLSs has not previously been studied.
The present study revealed that human HSP70 inhibited the IL-6, IL-8 and MCP-1 expression in RA FLSs induced by TNF-α stimulation (Figure 2), although HSP70 alone had no effect on FLSs (Figure 1). The findings suggest that self-HSP70 may have anti-inflammatory effects on RA, which might partly be due to downregulation of proinflammatory mediators by FLSs. However, HSP70 has also been shown to induce proinflammatory cytokines such as TNF-α, IL-1 and IL-6 in monocytes and macrophages [16, 43]. The explanation for these differences may lie in the different types and/or activation status of cells used for the experiments. Interestingly, similar extracellular anti-inflammatory functions for other self-HSPs have also been suggested. Treatment with human HSP60 of T cells in vitro was found to inhibit the production of proinflammatory cytokines TNF-α and interferon-γ, and to trigger production of the anti-inflammatory cytokine IL-10, which is mediated via Toll-like receptor 2 . Treatment of human monocytes with human HSP27 exaggerated IL-10 production . Treatment with human HSP10 in vitro inhibited lipopolysaccharide-induced activation of NF-κB; reduced secretion of lipopolysaccharide-induced TNF-α, IL-6 and RANTES (regulated on activation, normal T cell activated and secreted); and enhanced IL-10 production from human peripheral blood mononuclear cells .
These findings suggest that, rather than being proinflammatory, self-HSP reactivity might be a physiological mechanism for regulating proinflammatory responses and inflammatory diseases. It should therefore not be surprising if self-HSP70 were found to have an anti-inflammatory effect in RA. It has been shown that inflammatory stress induces HSP70 release from viable human FLSs and normal peripheral blood mononuclear cells . It is thus conceivable that, in the inflamed RA joint, inflammatory stress contributes to the expression and release of HSP70, and the extracellular HSP70 may act as a natural dimmer of inflammation, which might regulate both T cell and FLS mediated inflammation.
Thus far there exists no information about the signalling pathways that are involved in HSP70-mediated inhibitory effects on inflammation in FLSs. Signalling pathways that regulate proinflammatory mediator expression in RA FLSs include MAPKs and NF-κB. Three MAPK families have been implicated as playing a role in RA, including ERK1/2, JNK and the p38 MAPK . Interestingly, all three of these MAPK families are activated in RA synovial tissue and in cultured RA FLSs, and TNF-α has the potential to signal through all of them . Our study showed that treatment with HSP70 alone had no effect on activation of the three MAPKs (date not shown). However, human HSP70 markedly inhibited TNF-α stimulated p38, ERK and JNK phosphorylation (Figure 4). This inhibition was more obvious in TNF-α stimulated p38 phosphorylation. Collectively, these date suggest that HSP70 may suppress proinflammatory mediator production in RA FLSs via suppression of MAPK pathways.
Apart from the MAPKs, NF-κB is another key regulator of proinflammatory mediator expression and plays an important role in the induction of inflammatory cytokines in primordial mesenchymal cell lineages, including lymphocytes, macrophages and fibroblasts [35, 48]. NF-κB is mainly composed of two subunits – p50 and p65 – and is retained in the cytosol of nonstimulated cells by a noncovalent interaction with the inhibitory molecule IκB. Upon stimulation by proinflammatory cytokines such as TNF-α and IL-1, IκB is degradated and NF-κB is released and translocated to the nucleus to regulate inflammatory gene expression [35, 47]. NF-κB is also over-expressed in RA synovium [48, 49], and activated in RA FLSs in response to TNF-α and IL-1 [26, 47, 50]. We found that the degradation of IκB and subsequent nuclear translocation of the NF-κB subunit p65 induced by TNF-α were strongly inhibited by human HSP70. Accordingly, we conclude that the attenuation by HSP70 of proinflammatory mediator production upon exposure to TNF-α was at least partially mediated by the suppression of NF-κB pathway. The mechanism by which HSP70 inhibits the TNF-α induced degradation of IκBα remains unclear. We speculate that HSP70 in medium may bind its specific surface receptor on the RA FLSs, and activate intracellular anti-inflammatory signal transduction pathways such as JAK2-STAT3-SOCS3, which can inhibit the TNF-α induced degradation of IκB as well as subsequent activation and nuclear translocation of NF-κB. Recently, Human RA FLSs were shown to express high levels of the CD91 molecule , which is a known internalizing receptor for HSP70. Therefore, it is also possible that HSP70 may interact with the cell surface via the CD91 receptor, leading to receptor mediated endocytosis. Once taken up, HSP70 may function in the same way as does intracellular HSP70, which exerts its chaperone functions and inhibits degradation of IκBα.
In this study we demonstrate a novel role for exogenous human HSP70 in suppressing proinflammatory mediator production by human RA FLSs. The anti-inflammatory role played by human HSP70 in human RA FLSs may be accounted for by its ability to downregulate TNF-α induced activation of MAPK and NF-κB, two vital inflammatory signal pathways in FLSs of inflammation in RA. The results of this study indicate that human HSP70, which is upregulated and released in response to stress and inflammation, can function as a downregulator of FLS-induced inflammation in RA.
bovine serum albumin
enzyme-linked immunosorbent assay
extracellular signal-regulated protein kinase
glyceraldehyde 3-phosphate dehydrogenase
heat shock protein
c-Jun amino-terminal kinase
mitogen-activated protein kinase
monocyte chemoattractant protein
proliferating cellular nuclear antigen
tumour necrosis factor.
This work was supported by grant from the National Natural Science Foundation of China (30330280, 30671947), the National Basic Research Program of China (2007CB512007), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (20060533009).
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